This invention relates to a power monitor circuit and in particular to a circuit for monitoring the power of a personal computer.
Personal computers have circuits that monitor control the power supplied to different in parts of the computer. Some parts, such as memory, require a different voltage than other parts, such as the microprocessor. In order to conserve power and lengthen the life of the integrated circuits, it is economical to reduce the power available to components when the computer is inactive. Most computers have a power saving feature that reduces the power consumption after a predetermined idle time. The operator may have control of that time. During the power down time, minimal power is supplied to the computer. In theory, one only needs to supply enough power to sense when the user returns to resume usage (full power required). Despite the speed of integrated circuits, there remains a finite amount of time for the power supplies to reach their nominal operating levels. If the computer begins operation before the input power supplies reach their nominal operating levels, computations and operations performed by the computer may be erroneous. Such premature operation may cause errors in operation that, in return, could cause the computer to fail and shut down. Then, the user will have to restart the computer or perhaps repair it as well.
The power management feature contributes to overall efficiency, saves energy and reduces the cost of operating the computer. As personal computers become more sophisticated, the power up and power down monitoring circuits have likewise grown in sophistication. More sophisticated circuits are required because the computer uses numerous voltages. Some of the primary voltages used in a computer are 12 volts, 5 volts, and 3.3 volts. These are supplied from an AC/DC converter to other devices and chips within the computer. As such, the motherboard on a computer requires still further voltages derived from the primary voltages for operating memory chips, graphics chips and clock chips. Nevertheless, all of those derived voltages are derived from the three primary voltages of 12, 5, and 3.3 volts.
It is important that the various devices be powered up and powered down in the manner specified by the computer manufacturer. Unless the power up and power down operations are controlled and there is sufficient power, valuable data may be lost or the system may conflict with itself and crash.
For proper operation, the three primary voltages must be at or about 90% of their expected operating level. Microprocessor vendors, such as Intel Corporation, specify that the microprocessor and the motherboard will be fully operational after a predetermined time window. That time window is currently set to about 100 ms. In order to assist PC manufacturers, Intel also specifies that the 3.3 and 5.0 volt supplies must reach 90% of their value in less than 40 ms. The problem faced by computer manufacturers is how to monitor the primary voltages to determine when the voltages derived from the primary voltages can be created.
Some manufacturers have proposed using three power supply monitor chips, one for each primary voltage. That is a straightforward approach, but it multiplies the number of power supply monitors to match the three primary voltages. Still others have suggested using a single chip for monitoring the power supplies and on that single chip include three primary voltage monitor circuits, i.e., one circuit for each of the three primary voltages.